The present invention relates generally to integrated circuits, and particularly, but not by way of limitation, to a dynamic random access memory (DRAM) having a memory cell with a vertical access transistor with buried dual gates, and having buried bit and word lines.
Semiconductor memories, such as dynamic random access memories (DRAMs), are widely used in computer systems for storing data. A DRAM memory cell typically includes an access field-effect transistor (FET) and a storage capacitor. The access FET allows the transfer of data charges to and from the storage capacitor during reading and writing operations. The data charges on the storage capacitor are periodically refreshed during a refresh operation.
Memory density is typically limited by a minimum lithographic feature size (F) that is imposed by lithographic processes used during fabrication. For example, the present generation of high density dynamic random access memories (DRAMs), which are capable of storing 256 Megabits of data, require an area of 8 F2 per bit of data. There is a need in the art to provide even higher density memories in order to further increase data storage capacity and reduce manufacturing costs. Increasing the data storage capacity of semiconductor memories requires a reduction in the size of the access FET and storage capacitor of each memory cell. However other factors, such as subthreshold leakage currents and alpha-particle induced soft errors, require that larger storage capacitors be used. Thus, there is a need in the art to increase memory density while allowing the use of storage capacitors that provide sufficient immunity to leakage currents and soft errors. There is also a need in the broader integrated circuit art for dense structures and fabrication techniques.
The present invention provides an integrated circuit including a pillar of semiconductor material that extends outwardly from a working surface of a substrate. The pillar has a number of sides. A transistor is formed having a body region and first and second source/drain regions within the pillar. The transistor includes first and second gates that are each associated with a side of the pillar.
The invention also provides a memory device including an array of memory cells. Each cell includes a transistor. Each transistor includes a semiconductor pillar forming body and first and second source/drain regions. The transistor also includes first and second gates disposed adjacent to opposing sides of the pillar. The memory device also includes a plurality of substantially parallel first word lines. Each first word line is disposed orthogonally to the bit lines in a trench between columns of the memory cells. Each first word line allows addressing of first gates of the transistors of the memory cells that are adjacent to the trench in which the first word line is disposed. The memory device also includes a plurality of substantially parallel second word lines. The second word lines are interdigitated with the first word lines. Each second word line is disposed orthogonally to the bit lines in a trench between columns of the memory cells. Each second word line allows addressing of second gates of the transistors of the memory cells that are adjacent to the trench in which the second word line is disposed. A plurality of bit lines is provided, proximal to the substrate. The bit lines interconnect ones of the first source/drain regions of ones of the memory cells. In one embodiment, the pillars extend outwardly from an insulating portion of the substrate. In another embodiment, the pillars extend outwardly from a semiconductor portion of the substrate.
The invention also provides a method of fabricating an integrated circuit. According to one embodiment of the method, a substrate is provided, and a plurality of bit lines are formed on the substrate. A plurality of access transistors are formed on each of the bit lines. Each access transistor includes a first source/drain region shared by at least a portion of the bit line. Each access transistor also includes a body region and second source/drain region formed vertically on the first source/drain region. A plurality of isolation trenches are formed in the substrate, orthogonal to the bit lines. Each trench is located between access transistors on the orthogonal bit lines. A first word line is formed in a first one of the trenches. The first word line controls conduction between first and second source/drain regions of access transistors that are adjacent to a first side of the first trench. A second word line is formed in a second one of the trenches. The second word line controls conduction between first and second source/drain regions of access transistors that are adjacent to a first side of the second trench.
In one embodiment, the first word line also controls conduction between first and second source/drain regions of access transistors that are adjacent to a second side of the first trench. In another embodiment, the second word line also controls conduction between first and second source/drain regions of access transistors that are adjacent to a second side of the second trench.
In a further embodiment, another first word line is formed in the first trench, for controlling conduction between first and second source/drain regions of access transistors that are adjacent to a second side of the first trench. In yet a further embodiment, a second word line is formed in the second trench, for controlling conduction between first and second source/drain regions of access transistors that are adjacent to a second side of the second trench.
Thus, the invention provides high density integrated circuit structures and fabrication methods, such as for DRAM memory cell arrays and other semiconductor devices. Each memory cell can be fabricated in a surface area that is approximately 4 F2, where F is a minimum lithographic feature size. In one embodiment, a common first word line is shared by all of the access FETs that are located along both sides of the trench in which the first word line is located. In another embodiment, a common second word line is shared by all of the access FETs that are located along both sides of the trench in which the second word line is located. In further embodiments, split word lines are provided in either or both trenches. The split word lines provide separate addressing of gate regions of access FETs on opposite sides of the trench. Each of the unitary and split word line embodiments can be fabricated on a bulk semiconductor substrate, or on a semiconductor-on-insulator (SOI) substrate that results from using an SOI starting material, or by forming SOI regions during fabrication. The SOI embodiments provide greater immunity to alpha-particle induced soft errors, allowing the use of smaller storage capacitors.